Optical recording media have widely spread generally because of their superiority, such as high recording capacity and non-contact write/read system. Recordable optical disks, such as WORMs, CD-Rs, and DVD±Rs, record information by irradiating a very small area of the optical recording layer thereof with a focused laser beam to change the properties of the irradiated area and reproduce the recorded information making use of the difference in reflected light quantity between the recorded and non-recorded areas.
Compounds having an intense absorption, particularly an absorption maximum (λmax), in the range of from 550 to 620 nm are used as an optical recording material forming an optical recording layer of an optical recording medium, such as DVD-R.
There are many reports on indolium compounds having an indole ring for use as the optical recording material discussed above in view of their high sensitivity and possibility to respond to the increasing writing speed. For example, patent documents 1 to 6 listed below report styryl indolium compounds. Patent document 7 (see below) reports a low temperature decomposing cyanine compound having an indole ring and a benzyl group introduced into the 3-position of the indole ring. Patent document 8 (see below) discloses an indolium compound having a benzyl group introduced to the 3-position of the indole ring. Low temperature decomposing compounds readily form recorded portions (pits) in an optical recording layer and are considered suited for use in high-speed recording media. However, these materials are unsatisfactory in performance properties including heat resistance, light resistance, and recording characteristics.    Patent document 1: JP 11-34489A    Patent document 2: JP 11-170695A    Patent document 3: JP 2001-342366A    Patent document 4: JP 2002-206061A    Patent document 5: JP 2003-313447A    Patent document 6: JP 2003-321450A    Patent document 7: JP 2003-231359A    Patent document 8: JP 2006-150841A